The invention relates to a method of producing an electrically-conductive connection between enamelled wires, and a device for executing the method, as defined in the independent claims.
To produce identification systems, particularly transponders, transmitter/receiver units having logic circuits are coupled into integrated circuits (chips). The system is connected to the outside world by coils that operate in the radio-frequency range and are wound from thin copper wires insulated with a polyurethane-enamel covering. Typical wire diameters are about 10-50 .mu.m. Methods such as thermocompression welding are used to connect electronic components of this type to semiconductor chips, in which case the ends of the copper wire are connected to the metallized chip contact surfaces. The wire is severely deformed in this method, however, so there is a risk of breakage. Prior to the electrical connection, an enamelled wire is first stripped, particularly by means of micro-mills or overheated soldering baths, then tin-coated and bonded to the chip contact surfaces. Conventional chip contact surfaces often comprise thin aluminum films, which undergo a so-called gold-bump method in an additional processing step for contacting the copper wires; this renders the aluminum surface solderable, and permits an adhesion between the different materials.
Transponders of this type are known from, for example, WO-A-93 09551. This publication discloses the connection of enamelled wires and chip contact surfaces in a laser-welding method. In this instance, the wire ends are heated with a laser beam prior to the connection, and the enamel is melted and vaporized. For reliable contacting, it is necessary to gold-plate and/or tin-coat the chip contact surfaces. It has also proven advantageous to tin-coat the wire ends as well. The wire is deformed only slightly or not at all, which can avoid breakage of the wire; however, the contact surface is consequently not utilized optimally. In addition, this method necessitates a number of complicated process steps. Finally, the electrical connection can be worsened by melted residual enamel remaining at the wire end, or vaporized residual enamel that contaminates the wire end.
A method of connecting enamelled wires through ultrasonic bonding is known from JP-A-2-112249. First, the wire end is stripped and bonded to the connecting surface with simultaneous ultrasound and heating. Stripping takes place on a surface that is adjacent to the connection surface and has a surface structure resembling a washboard. There, with ultrasound, the wire end is stripped as with a grater until the metal wire is visible, and is subsequently drawn across the actual connection surface. A similar method, from the same Applicant, is disclosed in JP-A-2-54947. While the disadvantage of contamination of the connection surface by carbonized residual enamel is avoided, the production of the frictional surfaces at each connection surface is technically highly complicated. Furthermore, the heat effect required in the method to produce a permanent electrical connection represents an undesirable stress on the component. The conventional methods consist of a series of technically-complicated, time-consuming and costly process steps that increase production costs.
JP-A-6-61313 discloses a bonding tool that is especially Supplemental Page 2a (insert between lines 23 and 24 of p. 2)
DE-A-21 61 023 discloses a bonding method in which wires are bonded to contact surfaces with ultrasound. In the process, the wire is first fixed to the contact surface with an ultrasonic prepulse, and then welded to the contact surface with a more intense pulse. At this point, the wire is severely deformed; its thickness in the processed region is only about 30% of the original diameter. An arbitrary oxide layer or an insulating coating of the wire is broken open on both the top side and the contact side. The top side of the wire is unprotected, and insulation residue can remain on the contact surface. The enamel layer is preferably broken open on the underside of the wire, which is intended to come into contact with the connection surface, without damage to the enamel layer on the top side of the wire.
In a particularly-preferred embodiment of the method, both process steps are executed with ultrasound.
The advantage of the method is that a reliable contacting is attained without necessitating a prior process step of separate stripping and tin-coating of the wire end or a costly gold-plating or gold-bump method for the chip contact surfaces. In contrast to methods in which the enamel layer is removed all around the wire, only the enamel layer on the underside is removed. The remaining enamel layer can provide the electrical connection with long-term protection against corrosion without worsening the connection itself.
It is further advantageous that the stress in the connection region between the wire and contact surface is greatly reduced because, on the one hand, heating is no longer necessary in ultrasonic bonding and, on the other hand, the wire deformation is sufficient for an optimum contact resistance, but is small enough to prevent wire breakage.
The method is executed in two stages at the same wire end, and essentially at the same location on the connection surface. The ultrasonic tool requires no costly displacement unit for displacing the wire end during the connection process.
The method can be executed with conventional bonding machines that are modified to a small extent. It is advantageous to use a special bonding tool that has a simple design and can be used in conventional bonding machines, because the conventional bonding tool is not suitable for guiding loose wire ends from components.
In a preferred embodiment, the bonding tool is embodied so as to support the detachment of the enamel insulation on the wire underside, which is intended to produce the electrical contact, without damaging the enamel surface on the top side of the wire and, simultaneously, the tool advantageously deforms the wire such that an optimum wire surface can come into contact with the electrical contact surface.
In the first process step, a molten ball is successfully formed at the wire end, despite the enamel insulation of the wire, for producing a so-called "ball-wedge" contact. The permanent connection is effected in the second process step.
The features essential to the invention are listed in detail below and described at length with reference to figures. Shown are in: